1. Field of the Invention
The present invention relates generally to a viscous coupling apparatus incorporated in a power transmission system for automotive vehicles, and more specifically to an improvement in torque characteristics transmitted between two rotary members on the basis of difference in rotational speed between the two rotary members.
2. Description of the Prior Art
In the ordinary power transmission apparatus incorporated in a four-wheel drive vehicle of front wheel drive type, the two front wheels are directly driven by torque transmitted from a transmission, and the two rear wheels are driven by torque transmitted from a transmission via a viscous coupling apparatus. The prior-art viscous coupling is composed of a first rotary member connected to a transfer case, a second rotary member connected to a propeller shaft, a working chamber formed and partitioned between the two first and second rotary members and filled with a viscous fluid, a plurality of first annular resistance plates fixedly arranged at regular axial intervals on an outer circumferential surface of the first rotary member within the working chamber, and a plurality of second resistance plates also fixedly arranged at regular axial intervals on an inner circumferential surface of the second rotary member within the working chamber alternately to each of the first annular resistance plates.
In the above-mentioned power transmission apparatus, however, when the front wheel drive vehicle travels along a muddy road whose frictional coefficient is low, the front wheels tend to slip, so that a difference in rotational speed is produced between the front and rear wheels. As a result, the viscous fluid within the working chamber is sheared off by a great number of first and second annular resistance plates. Therefore, a torque is transmitted from the first rotary member connected to the transfer case to the second rotary member connected to the propeller shaft via the viscous fluid within the working chamber. When the four wheels including the rear wheels are rotated in 4 wheel drive mode in the front wheel drive vehicle, the vehicle can easily get out of the front wheel slipping conditions.
In the prior-art viscous coupling apparatus as described above, however, the relationship between the difference N in speed between the first and second rotary members and the torque T transmitted to the propeller shaft (i.e. rear wheels) is such that as shown by A in FIG. 4, in which T first increases with increasing N but gradually saturates. Therefore, there exists a problem in that it is impossible to transmit a large torque to the rear wheels in a short time, so that it is impossible to quickly allow the vehicle to get out of the front wheel slipping conditions.
In addition, since the torque T increases with increasing rotational difference N between the two rotary members along a curved line, there exists another problem in that the riding comfortability is not preferable when the vehicle gets out of the slipping conditions, because it has been known that the riding comfortability is good when the torque T increases with increasing rotational difference N between the two members in accordance with linear characteristics.